This invention is related to printed circuits for use in electrical and electronic equipment. In particular, it is an improved method of making printed-circuit boards and of producing printed circuits on surfaces other than conventional printed-circuit boards. This invention is also concerned with making printed circuits and is especially concerned with a method of making a printed circuit in which a layer is applied in a desired pattern to a substrate.
An element that is common to almost all electronic equipment is the printed circuit, typically in the form of a printed-circuit board (PCB). A PCB is generally made by laminating copper foil to a board. A desired pattern that includes conductors in the plane of at least one surface of the board is placed upon the board, and holes are drilled or punched for the mounting of components. This pattern is typically realized by placing a photosensitive resist on the board, exposing a photograph of the desired pattern on the resist, and developing the resist to produce a protective coating over the pattern. The remainder of the resist is treated to remove undeveloped resist from portions where it is desired to remove the copper. The board is then treated with a process that removes the exposed copper. When the remaining resist is then removed, the desired pattern remains in the copper. Mounting holes for components are drilled or punched at desired locations somewhere in the course of this process, either before or after the removal step.
The materials most commonly used for PCBs are either polymerized epoxy materials containing glass fibers or paper bound by impregnated synthetic resins such as phenolic materials. The latter group is often referred to generically as synthetic-resin-bonded paper (SRBP). Boards made of either of these types of materials are clad with copper on one side or both sides, with the heat and pressure of the cladding or laminating process helping to cure the resin. The material cost of an SRBP PCB is typically about half as much as that of an epoxy fiberglass board of the same surface area, so there is a potential cost advantage when the SRBP board can be used. There are various bases for the selection between epoxy fiberglass and SRBP material. For example, the epoxy fiberglass boards are generally higher in strength and are preferred for use in equipment that may be subject to vibration.
A particular problem of circuit design that leads to complication in PCBs is the fact that not all electronic circuits can be made with their connections in a single plane. It is sometimes necessary to make bridging connections between different portions of a circuit. A considerable amount of ingenuity goes into the design and layout of PCBs to minimize such bridging connections. However, sometimes it is impossible to avoid them. In such a case, it is possible to solder jumper wires between the portions of the circuit that are to be connected. This is especially undesirable for long runs, and it is better avoided even for short runs. A better solution is to use PCBs that have more than one conducting layer. The simplest of these is a two-sided PCB. This is a board that has copper laminated to both sides. Separate patterns are etched on the two sides to effect the desired circuit layout and cross-connections. However, in order to make such cross-connections, and to complete the connection between the two sides of the board, it is normally necessary to use epoxy fiberglass because of the necessity of plating holes through the board to connect the top layer to the bottom layer. Holes that are drilled in an SRBP board are adequate to support the leads of components that are placed in the board for soldering, but they are not normally clean enough when drilled or punched to permit satisfactory electroplating of connections between layers of the board. There is thus a long-felt need in the PCB industry for a way of making PCBs with crossover connections on single-sided SRBP boards without using soldered jumper wires.
The same considerations apply equally as much to boards having more than two layers. These boards, referred to as multilayer boards, are often used in more complicated circuits where one set of bridging connections is not enough. As with the two-sided board, it is normally necessary to use epoxy fiberglass for such PCBs. It can be seen that the result of a need for crossed connections in the typical etched copper PCB leads to an increased cost because of the need for epoxy fiberglass in the PCB instead of the less-expensive SRBP boards.
The problems just described become extreme in the case of PCBs for keyboards. A keyboard for a typewriter, computer or the like typically generates an electrical signal when a key is depressed to make an electrical contact or an inductive or capacitive coupling. Such a coupling is made directly or indirectly between two separated conductors on the board or on a flexible plastic membrane that is spaced apart from the board and makes a conductive or field contact when pressed toward the board. When the keyboard is the typical typewriter or computer keyboard, it is impossible or nearly impossible to avoid crossed connections. The physical size of the hands of an operator also sets a limit to the minimum size of a PCB for a keyboard, since it is necessary to place a keyboard switch on the PCB at a location directly beneath the key to be depressed. As a result, the typical PCB for a keyboard is of the order of ten to eighteen inches by four to seven inches (25-45 cm. by 10-18 cm.) This size requirement has caused the PCB to become a significant part of the cost of a typewriter or computer keyboard. It is not normally possible to use an SRBP PCB because of the need for crossovers and the attendant plated connecting holes. The result is a relatively expensive epoxy fiberglass PCB, laminated and etched on both sides. This cost could be greatly reduced if it were possible to use an SRBP board that contained a circuit on only one side.
A second problem in the manufacture of PCBs for keyboards is the fact that keyboards either have pairs of exposed electrical conductors that are bridged by another conductor or coupled capacitively or inductively to make an electrical connection when a key is depressed, or else have flexible membranes that couple to the board when pressed. It is necessary to apply some form of protection to the exposed electrical conductor so as to minimize the buildup of corrosion that would interfere with the making of the electrical connection. This is most commonly done by etching a copper pattern of interlaced combs, parallel conductors or the like and plating gold to the combs to provide a contact surface that is conductive electrically and that is not readily corroded by exposure to the atmosphere. Gold may be plated to the copper either by electroplating or by electroless plating. Either of these represents an additional element that contributes to the cost of preparing a PCB for a keyboard.
The usual intended use of a PCB is to serve as a mount for components such as resistors, capacitors, diodes and transistors. Any of these components is typically inserted by placing its leads into holes in the PCB which is then passed through a wave-soldering process to attach the components physically and electrically to the PCB. During the process of manufacturing the PCB, the board is typically coated in part with an organic polymer solder resist to prevent solder from adhering to the covered regions. If the PCB is one designed for a keyboard, the resist is typically deposited so as to cover conductors on the keyboard surface but is masked to leave the comb or other keyboard switches exposed for operation.
A use that is analogous to that o f PCBs is the manufacture of electrical cables or the like by depositing conducting material on flexible plastics such as mylar. This is often done by some form of printing process such as screen printing . Most, if not all, of the materials, typically plastics, that are used for flexible cables or flexible flat conductors are not adapted for wave-soldering, and it is therefore necessary to make compression connections or the like at the ends of the cable or flat conductor. As a result, there is no way to attach resistors or capacitors to flexible material, and a rigid PCB is therefore used with the flexible material to serve as a component mount.